The present invention relates to new and useful polymers, and more specifically, to improved nonleachable, optically transparent, radiopaque barium-containing polymer complexes, compositions of matter and articles made therefrom.
Translucent polymeric materials, and particularly, acrylic type resins have been widely used for year in both medical and dental applications. In dentistry, for instance, plastics are used in a broad range of materials and appliances, including removable dentures, temporary crown and bridge materials, restorative materials, impression materials, and the like. They also find many applications in medicine, such as surgical and body implants and other prosthetic devices; heart valves, blood vessels, etc. Translucent plastics are also widely used in medical appliances, such as catheters.
The desirability of imparting radiopacity to plastics used in dentistry and medicine has been recognized. In dentistry, for example, it has been difficult to detect secondary caries or underlying decalcified dentin resulting from the placement of unreinforced direct restorative resins because these materials are relatively radiolucent, and are not opaque to x-rays. Surveys have also shown that dental instruments, materials and nonfixed appliances become embedded in soft tissues, and also become ingested or inhaled inadvertently by patients Although incidences of ingestion or inhalation of dental plastics are relatively rare compared with other foreign objects, the occurrence may result in a severe medical emergency or even death. The potential severity of such an incident makes it imperative to diagnose and remove such foreign bodies rapidly.
In medicine, hip joint replacements require cementing. Therefore, it would be desirable to monitor the positioning of bone cement without surgical procedure. Similarly, the ability to monitor by x-ray replacement heart valves, arteries, including the path of catheters traversing blood vessels and organ systems would be desirable. Hence, there is a need for polymeric materials with increased radiation absorption potentials which also possess the requisite properties for safe and acceptable use in dentistry and medicine.
Heavy metal salts, like the salts of barium, and particularly barium sulfate, have long been used as contrast medium in diagnostic radiography. It has properties which would suggest its suitability for increasing the radiation absorption potential of medical and dental resins. As a result, substantial effort has been made to incorporate barium sulfate and other barium salts, like barium bromide, barium chloride, barium fluoride, barium nitrate, etc., into polymers to render them opaque to x-rays. However, earlier radiopaque polymers containing barium have not been totally satisfactory. Barium based radiopaque materials fall into two principal groups; radiopaque glasses containing imbedded barium metal, and polymers mixed with barium salts. In the case of barium imbedded into radiopaque glasses, the barium is not molecularly bound to the polymer matrix, and therefore, has a tendency to weaken the composite. Moreover, because glass filler based resins lack homogeneity a further weakening of regions in the matrix results. Those regions of a composite having little or no glass are radiolucent. In addition, a light scattering effect is produced by radiopaque glasses which alters optical properties.
Heretofore, polymers with added inorganic barium salts were essentially physical mixtures, present as fine powders locked in a matrix. Their preparation resulted in an uneven distribution of the salt, and had an adverse affect on the mechanical properties of the plastic material. The salt gradually leached out of the matrix causing discoloration of the polymer and release of heavy metal toxins. The salt and polymer remained as separate heterogeneous phases producing an opaque, cloudy material which scattered light. Mixing does not impart homogeneity between the salt and polymer.
Examples of heterogeneous barium salt containing polymers are disclosed by E. C. Combe in the Dental Practioner, 51-54 (1971); Journal of Dental Research Supplement, Vol. 50, 1192 (1971); Journal of Dentistry, Vol. 1, 93-97 (1972); British Dental Journal, 355-358 (1969); Journal of Dental Research, 668, (May-June 1971). Combe reported in the Dental Practioner, Dental Research and Dental Research Supplement Supra the preparation of poly (barium acrylate) and methyl methacrylate-barium acrylate copolymers. The polymers were extensively crosslinked by the divalent barium ions. The barium polymers were incorporated into a conventional acrylic dough and heat cured. Combe concluded that the addition of barium-containing polymers downgraded the mechanical properties of his modified test materials. Crosslinking made the barium-containing polymers harder and more brittle causing a reduction in impact strength, and increased the quantity of monomer needed to form a dough of satisfactory consistency. Combe's later work, reported in the Journal of Dentistry, compares the properties of radiopaque denture base materials prepared with barium sulfate and barium fluoride mixed with poly (methyl methacrylate). Combe used an organic silane to aid in bonding the barium salts to the acrylic. Heat cured doughs were prepared with polymers having 8 percent w/w barium sulfate, 10 and 20 percent w/w barium fluoride and their mechanical properties tested. Combe concluded that none of the materials containing the barium based polymers were as strong as the unmodified acrylics.
Additional radiopaque contrast media have been formed by the use of (a) other heavy metal salts, such as bismuth, silver, and lead; (b) heavy metals imbedded in silica filler and then added to the resin composition, and (c) the use of highly halogenated polymers. Halogenated polymers or halogenated organic additives, e.g. brominated polymers have good physical properties, but the halogen functional groups tend to hydrolyze or decompose and form leachable compounds. This is especially pronounced in acidic aqueous fluids as commonly found in humans. In addition to the potential toxic effects, leaching of bromide and other halogen causes discoloration of the resin which will gradually convert the polymer to a radiolucent material. The halogenated organic additives can also act to decompose the polymer. Accordingly, there is a need for improved barium-containing radiopaque materials where the barium is molecularly bound to the polymer to form clear, homogeneous, nonleachable materials with mechanical properties which are substantially equivalent to those polymers which are free of barium.